<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <style>
        *{margin: 0;padding: 0;}
        :root, body{height: 100%;}
        body{
            display: flex;
        }
        .box1, .box3{
            width: 100px;
            background-color: skyblue;
        }
        .box2{
            background-color: violet;
            flex: 1;
        }
    </style>
</head>
<body>
    <div class="box1"></div>
    <div class="box2"></div>
    <div class="box3"></div>
</body>
</html>